Mfcttrf

I've seen quite a few news about astronomy but I'm not deep into knowing how it works or how people observe it. It got me thinking: if I want to watch a supernova for example, how long will I have to look for it to watch the whole "event"? Or events like NASA "seeing something coming out of a black hole", were they "looking" at the blackhole when something came out really quickly or did they observe it for months? Or maybe the birth of a blackhole?

Now, I do know black holes that quite a few trillion of years to vanish, I'm just curious how much time does it take for astronomers to observe such events.

As described in other answers, most astronomical events vary over rather long time scales. Even a supernova, which starts suddenly, has a subsequent light curve that peaks, then decays usually over many months. However, there are some shorter events.

In the recent LIGO gravitational wave observations, the final stage inspiral and merger of binary black holes or neutron stars generates a detectable GW signal for less than a second.

Other shorter term astronomical observations occur within the Solar system. Of course, the Sun and planets move across the sky over the year, and the Moon changes its phase continuously, repeating about every month. Occasional Solar and Lunar eclipses occur over a time scale of hours, with totality lasting only a few minutes for a Solar eclipse. Similiar time frames apply to other Solar system transits. Occultations, where a solar system object passes in front of a star, are nearly instantaneous for a visual observer. Meteor trails in the atmosphere are common, and are rarely visible for more than a few seconds.

A supernova (and many other interesting events) starts abruptly -- a flash of neutrinos lasting just a fraction of a second, but then different aspects of the event play out over minutes, days, hours or years. The astronomy community has for some years been developing increasingly sophisticates ways to deal with such events early on. Instruments with a very wide field of view (gravitational observatories, neutrino observatories, gamma ray detectors on satellites) detect the events initially and bring first networks of robotic telescopes and then humans and still larger telescopes into the loop to try and get as much information as possible about the violent and fast developing early stages. See for example http://growth.caltech.edu/

Other things are slower. The "something coming out of a black hole" (not really that, just something coming from very close to a black hole) event takes a long time, this was just the first occasion when they'd observed that black hole with the right instrument.

In our galaxy, supernovae visible from earth are very rare events,so if you wait for astronomers to raise the alarm when they see the next one you might have to wait hundreds of years. The prospects of seeing one in faraway galaxies are much better,but even so they are not everyday events. The chances of seeing one from the very beginning when the star collapses and explodes are almost non-existent,because when they occur they have to reach considerable brightness to become noticeable from Earth. However, searching for supernovae is something a well equipped amateur can do, but it requires a lot of patience. As for the energetic particle beams that some black holes send out, they don't arise from within the black hole itself but from the matter in the accretion disc which is accelerated by the poles of the black hole's magnetic field. As this process continues for many years,you don't need to react so rapidly as a supernova discoverer,but you would need some very high tech equipment to observe and photograph it (the particle beams eject large amounts of matter which form large swirls,sometimes light years in length,from both poles of the black hole, and need a radio telescope to image them).